Because of their value to industrial chemical processes, many catalysts and catalytic processes have been developed over the years. One important class of catalysts is heterogeneous catalysts which can generally be classified as either supported metal catalysts or massive metal catalysts, also sometimes referred to as bulk metal catalysts.
Supported metal catalysts are usually comprised of relatively small metal crystallites on relatively large support particles. The support particles may also act as a promoter for the active ingredients of the catalyst. Because the metal crystallites are small compared to the support particles, the effective metal surface area per unit volume of catalyst is relatively small, thus limiting the potential activity of the catalyst.
In contrast, massive metal catalysts, such as the well known magnetite or spinel-based ammonia synthesis catalysts, or Raney metal type catalysts, have metal domains having dimensions which are usually far greater than any promoter phase particles or promoter species which may be present. The effective metal surface area per unit volume of catalyst is also relatively small limiting the potential activity of the catalyst. To maximize the effective catalytically active surface area per unit volume, it would be desirable to produce a catalyst having a median metal crystallite, or domain, size of about 25 .ANG. to about 500 .ANG. in diameter. The desired crystallite size, of course, depends on the intended application. For example, crystallite sizes in the range of 25 .ANG. to 250 .ANG. are desired for catalyzing reactions involving relatively small substrate molecules, while larger crystallites, in the range of 250 .ANG. to 500 .ANG., are desired for catalyzing reactions involving large substrate molecules. By effective metal surface area per unit volume of catalyst we mean metal area accessible to reactants. Because of the demanding nature of certain catalytic reactions (e.g. ammonia synthesis) the surface area of very small metal crystallites may be either ineffective or non-selective.
Although it is possible to conventionally produce supported metal catalysts containing metal crystallites having a median diameter within the 25 .ANG. to 500 .ANG. range, it can only be done with relatively large support particles, or a relatively large pore support, thus the effective surface area suffers and a catalyst is produced having less activity then it would otherwise have if the support particles were smaller.
One commercially significant route of catalyst preparation is by precipitation methods. For example, U.S. Pat. No. 4,251,672 teaches a method for preparing nickel hydrogenation catalyst precursors by coprecipitation of a catalyst support and the catalyst from aqueous solutions at or near the boiling point of the solutions. Such a process usually employs a seeding agent such as kieselguhr to form a suspension. After reduction the resulting catalyst usually consists of active metal on relatively large support particles. Another representative disclosure of precipitation methods to produce catalysts, particularly ammonia synthesis catalysts, in U.S. Pat. No. 3,840,479 which also utilizes a suspension of a supporting material. It is disclosed in that patent that iron must, by necessity, be present in the bivalent state prior to precipitation, which precipitation is carried out at a pH of about 2 to 6.5.
Precipitated iron catalysts for ammonia synthesis have also been described recently by Klisurski, Mitov, and Tomov (Stud. Surf. Sci. Catal. 16 (1983) 421). The process described involves forming a solid solution of Al.sub.2 O.sub.3 in .alpha.-Fe.sub.2 O.sub.3 by heating to 800.degree. C. in air coprecipitated iron and aluminum hydroxides. The resulting crystalline masses were characterized by X-ray diffraction and the reduced catalytic materials had surface areas in the range of about 1 to 30 m.sup.2 /gm. They showed that, with addition of appropriate promoters, the catalytic activity for ammonia synthesis of these catalysts was comparable to or slightly higher than that of a conventional industrial catalyst. The catalyst precursors of the present invention are amorphous rather than crystalline and after reduction have surface areas in the range of 10 to 150 m.sup.2 /gm. Further, the catalysts of the present invention have ammonia synthesis activities substantially greater than conventional industrial catalysts.
A new class of catalyst, which is referred to herein as dual colloid catalyst are disclosed in a copending patent application and U.S. Ser. No. 639,439 filed on the same day as this application, which disclosure is incorporated herein by reference.
Although several techniques have met with varying degrees of commercial success for producing catalyst compositions, there still exists a great need in the art for new techniques of producing catalyst compositions, such as the dual colloid catalysts.